The overall goal of this proposal is to improve long-term survival of organ transplants with particular emphasis on lung transplantation. This is particularly relevant to the mission of the agency since 5 year survival (40-50%) of lung transplants is far below what is seen for other solid organ transplants. Acute and chronic rejection/bronchiolitis obliterans (BO) are major limiting factors for long-term lung allograft survival with chronic rejection coming from progression of the profibroproliferative process and the lung's relative inability for developing transplant tolerance. We have recently shown that indoleamine 2,3-dioxygenase (IDO) results in protection against acute rejection using a rat orthotopic lung allograft model. IDO is an inducible enzyme of tryptophan metabolism with immune modulating and potentially anti-fibrotic properties that may provide long-term graft survival effects. In this proposal, we plan to further evaluate IDO's role in protecting not only against an acute rejection, but also against the development of transplant mediated fibrosis as an indicator of chronic rejection. There are two major aims for this proposal. The first aim will determine whether the non-viral gene system (Sleeping Beauty transposon/transposase) generates long term IDO expression and protection of rat lung transplants. We will examine if IDO's graft survival properties are mediated by non-immune mechanisms in preventing transplant associated fibrosis through its interaction with profibrotic cytokines and inhibiting fibroblast proliferation. The second aim will determine whether IDO inhibits the alloimmune phase of rejection by promoting transplant tolerance. We will examine if augmented IDO in lung allografts 1) results in a tolerant phenotype in vivo 2) maintains dendritic cells in an immature/tolerant state 3) promotes the development of T regulatory/suppressive cells 4) if target cells with IDO expression are protected against cytotoxic T cells. Lung transplantation is an accepted therapy for patients with end-stage lung disease. Unfortunately its success has been limited by the development of chronic rejection. This project has significant relevance to public health since IDO with its ability to prevent transplant-associated fibrosis and promote acceptance (tolerance) of the lung transplant in which both would enhance long-term survival makes it a potential therapeutic option in lung transplantation.